Water-added and kneaded, wet coating formulations have conventionally been used as finishing materials for interior walls. A variety of materials are used as principal materials in wet coating formulations, but from the viewpoint of the fireproofness and comfort of formed interior walls, gypsum-based coating formulations are widely used. A gypsum-based coating formulation is generally commercialized as a dry gypsum composition, which contains calcined gypsum (in particular, hemihydrate gypsum) as a principal component and also contains various additives added as needed for the formation of a wall surface provided with functionality such as external attractiveness or moisture absorption/desorption properties. At a working site, the dry gypsum composition is added with water and kneaded, the resulting gypsum-based coating formulation obtained in a coatable form (hereinafter also called a “gypsum-based wet coating formulation” or “wet coating formulation”) is coated on an underlayment such as a gypsum board by using a tool such as a trowel, and after setting and drying, a wall surface is completed.
With a view to reducing so-called sick building syndrome or the like which has become a social problem in recent years, a variety of materials (additions) are under study to make improvements in building materials. Gypsum-based coating formulations also include those equipped, for example, with an adsorbing function for formaldehyde, which is considered to be a cause of sick building syndrome or the like, by contriving its additives. A proposal has also been made on a gypsum-based wet coating formulation that owing to the addition of rosin, aluminum sulfate and the like, enables the formation of a smooth and high-accuracy, coated wall surface by single coating work (see Patent Document 1).
Concerning gypsum-based coating formulations used as finishing materials for interior walls, on the other hand, dry gypsum compositions, which are used after adding various pigments, respectively, to impart external attractiveness, are provided as commercial products. Wall surfaces formed with these materials present finished surfaces having a wide variety of colors. The addition of a pigment, which is needed in such a case, is generally conducted by a method that adds the pigment as needed at a working site. However, this method requires cumbersome work, and moreover, is difficult to evenly mix the pigment. Especially with a gypsum-based coating formulation which is basically white, the mixed state of its pigment has a high tendency of affecting the finished quality of an interior wall or the like. Accordingly, it is also practiced to provide, as products for gypsum-based coating formulations, dry gypsum compositions with various pigments added, respectively, at factories. These products make it possible to avoid the cumbersomeness of mixing pigments at working sites, and moreover, the use of these products makes it possible to realize the formation of evenly-colored interior walls free of color irregularity because they are provided as products with the pigments mixed stably and evenly.
Color irregularity may, however, occur after coating or dry finishing even when a gypsum-based coating formulation is prepared by adding water to a dry gypsum composition as a product with a pigment added beforehand. The present inventors, therefore, have come to recognize a problem that the high external attractiveness required for interior walls cannot be realized for the above-mentioned color irregularity.
In the meantime, a proposal has been made on a conventional finishing material, which contains an inorganic powder having particle sizes of greater than 0.1 mm, such as clay, silica stone powder and diatomaceous earth, and gypsum and is useful for forming clay walls. According to the proposal, a pigment itself is provided with improved color developing properties to avoid color irregularity on a finished surface (see Patent Document 2). Another proposal has been made on a hydraulic, color-finishing material composition containing alumina cement, gypsum and blast furnace slag. According to the proposal, a setting adjuster is added. The addition of this setting adjuster is described to provide not only excellent fast-setting properties, working characteristics and setting characteristics but also superb staining properties, and also to conduct both surface preparation of a floor underlayment and color finishing at the same time (see Patent Document 3).
These materials both contain gypsum as a filler, but their principal component is the inorganic powder, alumina cement or the like. In these proposals, no study was, therefore, made about the color irregularity on a coated surface formed with a dry gypsum composition which contains gypsum as a base material and is useful as a finishing material for an interior wall, although the present invention is concerned with such color irregularity. Further, a wet coating formulation which contains gypsum as a base material is basically white close to pure white. Therefore, the problem of its color irregularity cannot be equated with the color irregularity of conventional color-finishing materials containing as a principal component such a material as described above, and requires a new technical study. In other words, compared with the above-described materials of the conventional technologies, a wet coating formulation which contains gypsum as a base material is totally different in the color of the base material, and is also different in its setting characteristics and the like (working time and the like). Therefore, the wet coating formulation is obviously required to lessen this problem of the occurrence of color irregularity while satisfying working characteristics, setting characteristics and the like inherent to gypsum, to say nothing of the difference in the meaning of color irregularity. Incidentally, the term “working time” as described above means a time during which one prepared as a reaction-type, gypsum-based coating formulation by adding water to a dry gypsum composition and kneading the resulting mixture to permit coating on a wall can be subjected in a good state to coating work with a trowel or the like.